Receivers for a handset or headset are conventionally designed using a lumped parameter model. Using this model, the speaker diaphragm behaves as a piston, as discussed for example in Theory & Design of Loudspeaker Enclosures by J. E. Benson; (1996) Howard W. Sams & Co. ISBN 0-7906-1093-0 and High Performance Loudspeakers, by M. Colloms, (1997) John Wiley ISBN 0-471- 97089-1. In section 7.11 of Acoustics (L. L. Beranek; (1954) Acoustical Society of America (1996); ISBN 0-88318-494-X), Beranek recognises that the typical loudspeaker has a modal behaviour but concludes that no “tractable mathematical treatment is available by which the exact performance of a loudspeaker can be predicted . . . ”. Beranek concludes that loudspeakers must be designed to minimise the diaphragm modal behaviour to permit the assumption that the cone of the diaphragm moves as a unit. Numerical methods such as Finite Element Methods (FEM) or Boundary Element Methods (BEM) allow accurate computation of the diaphragm vibration field and the associated acoustic radiated field.
Telephony receivers are designed to provide a frequency response for the traditional telephony band of 300-3000 Hz coupled to an ITU-P.57 type 1 (IEC318) artificial ear. Some telephony receivers have been designed for a low impedance-type artificial ear but again, only for the traditional telephony band. In conventional telephony receiver designs, the exact position of leaks or resonators with respect to diaphragm geometry does not matter. Measures to counter the modal behaviour of the diaphragm result in the use of filters that generally make the device large and inefficient.
The prior art provides insight into the approaches taken to counter modal behaviour, but none of the prior art explicitly exploits the modal nature of the diaphragm.
For example, U.S. Pat. No. 5,953,414 to Abraham & Dufossé discloses a piezoelectric speaker capsule for a telephone handset with leaks behind the diaphragm and a Helmholtz resonator to build the high frequency end of the earpiece. Due to the rigidity of this type of diaphragm, only one mode, namely the piston mode, is used and the rear cavity cannot block the diaphragm significantly to modify the vibration frequency. The speaker capsule is not designed to make several vibration modes appear in the frequency range of interest, even if leaks are provided to dampen the vibration field amplitudes of the disk.
U.S. Pat. No. 5,729,605 to Bobisuthi et al. discloses several configurations for adjusting the frequency response by varying various design parameters such as rear cavity size and leaks or front resonator leaks. Bobisuthi et al. discloses designs based on the lumped parameter model. Thus, the particular location of holes with respect to the diaphragm does not matter. Further, hole locations that excite unwanted modes are compensated for by adding or modifying a resonator.
WO 99/35880 (Williams & Mercer) discloses a low impedance-type earpiece. Using a front resonator, the diaphragm is uniformly loaded and thus the device is less sensitive to externally applied acoustic load. This design is again based on the simplified lumped parameter model.
U.S. Pat. No. 5,058,154 to Morten discloses a low acoustic impedance earpiece based on a pure acoustic ohmic connection between the diaphragm and the exterior of the handset. The path can be any shape but must have some sound damping material in it. This earpiece is designed based on the traditional model in which the diaphragm is modelled as a piston and the acoustic damping material is provided to deal with the unwanted diaphragm mode.
U.S. Pat. No. 5,784,340 to Kanai discloses a piezoelectric earpiece in which acoustical design considerations are limited to the resonator volume which is damped by a mesh and one or more holes.